Killer factor is a cytoplasmically inherited genetic determinant in the yeast Saccharomyces cerevisiae that confers upon its host cells ("killers") the ability to secrete a toxic polypeptide that kills sensitive cells. Killer factor is associated with two double-stranded RNAs (dsRNAs), which are found cytoplasmically separately encapsidated in virus-like particles (VLPs). These two dsRNAs are L, of 3.0 x 10 to the 6th power daltons, which encodes the major VLP capsid protein, and M, of 1.3 x 10 to the 6th power daltons, which encodes the toxic polypeptide. There are also strains with smaller, or S, dsRNAs encapsidated in defective interfering VLPs. The VLPs have a capsid associated RNA polymerase activity. Knowledge of the 5' and 3' end sequences of the killer factor RNAs is prerequisite to understanding their replication, transcription and translation. We presently know the ultimate 5' nucleotide of L and M and the sequences of the 3' T1 oligonucleotides of L, M, and one S. We will extend these sequences by the gel sequencing technique applied to 5' and 3' end labeled RNAs. The relationship between several S dsRNAs and their parental M dsRNA has been established by two-dimensional gel electrophoresis of T1 oligonuculeotides. These defective interfering particle RNAs preserve some of their parental T1 oligonucleotides but diverge from their original sequence with time. The product of the VLP capsid RNA polymerase has been characterized as single-stranded and probably the transcript of one of the two strands of the VLP dsRNA. Several types of sequence analysis are being applied to the product of the in vitro reaction.